The invention concerns a membrane reactor for the conversion of methanol into CO- and CO.sub.2 - free hydrogen.
Such reactors are considered for use in connection with fuel cells, particularly with PEM fuel cells. The latter are being developed for future use as components of electrical vehicle drive systems.
Compared with other fuel cells, PEM fuel cells have the advantage that they use a polymer solid electrolyte which facilitates the handling and the construction of compact fuel cells. PEM fuel cells have a high power density of about 1 W/cm.sup.2 at temperatures of 80.degree. C. For the oxidation of pure hydrogen in acid electrolytes such as in a PEM fuel cell, Platinum (Pt) has been found to be the most effective electrocatalyst. However, since the infrastructure present for the operation of motor vehicles should also be useable in the future, it should be possible to use liquid fuels that is liquid methanol. Liquid methanol, however, must be converted in the motor vehicle to hydrogen by a reforming reaction. During the conversion of methanol to hydrogen, side-products such as CO are generated which act as catalyst poisons for the electrocatalyst Pt. As a result, the power output of the fuel cell is drastically reduced if the fuel gas, that is the hydrogen, includes CO.
Consequently, the fuel gas generated by the reformer has to be treated before it is supplied to the PEM fuel cell so as to reduce the CO content to less than 10 ppm. At present, the desired purity of the fuel gas can be achieved only by using a Pd/Ag membrane. But such Pd/Ag membranes are very expensive.
In order to provide for the required purity, it is also possible to chemically convert the CO with hydrogen to methane. (methanization reaction). With the use of precious metal catalysts, it is possible to reduce the CO content in such a gas treatment unit to 10 ppm at low reaction temperatures (180.degree. C.). It is however first necessary to remove the CO.sub.2 from the gas mixture. Under corresponding reaction conditions CO.sub.2 is also subject to the methanization reaction or it is converted to CO at slightly higher temperatures.
It is the object of the present invention to provide a reactor which converts methanol to hydrogen in such a way that the hydrogen can be utilized directly as combustion gas in a PEM fuel cell.